The present application claims priority to Japanese Application(s) No(s). P2001-158527 filed May 28, 2001, which application(s) is/are incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a liquid crystal display device widely used in, for example, a notebook-size personal computer, a portable terminal, a portable video tape recorder (VTR), or a digital still camera. The present invention is one related to a liquid crystal display device including a reflection function and a transmission function among the different types of liquid crystal display devices. When the liquid crystal display device has a reflection function and a transmission function, it has excellent visibility both indoors and outdoors. Accordingly, such a liquid crystal display device is suitable for use in, for example, a portable terminal, a portable VTR, or a digital still camera.
2. Description of the Related Art
Various types of flat displays where the content of display can be rewritten by modulating light using an electrical signal have been developed and produced. Self-luminous display devices which have been researched and developed a great deal in recent years include OLED (organic light emitting diode), a plasma display, and an FED (field emission display).
A self-luminous display has excellent visibility indoors compared to a liquid crystal display because the self-luminous display is brighter than the liquid crystal display. However, since the outdoor environment is often brighter than a self-luminous display disposed outdoors, the visibility of the self-luminous display device becomes considerably poor in such an environment.
In addition, in a self-luminous display, electrical current and electrical voltage required to drive it are relatively large, so that electrical power consumption thereof tends to become large. When electrical power consumption becomes large, the display device is not suitable for use in, for example, a portable terminal of a digital camera, a portable VTR, a cellular phone, or the like.
On the other hand, a reflective liquid crystal display device uses external light rather than light generated by itself for a displaying operation, so that its visibility becomes good due to increased brightness, rather than becoming poorer even when the reflective liquid crystal display device is used outdoors where there is sufficient brightness. In addition, since, unlike a transmissive liquid crystal display, a reflective liquid crystal display does not need a backlight, electrical power consumption thereof is small, so that it is most suitable for use in small portable devices.
However, although a reflective liquid crystal display device has excellent visibility in a bright outdoor environment, its visibility obviously becomes poor when it is used in a dark environment at night because it does not generate light. This occurs naturally in a reflective liquid crystal display because the brightness of the surrounding environment is directly reflected in the brightness of the display.
Accordingly, a display device of, for example, a portable terminal needs to provide excellent visibility in any environment, both indoors and outdoors. One type of display device for achieving this object is a transmissive-and-reflective liquid crystal display device. In general, the transmissive-and-reflective liquid crystal display device is realized in the following two ways.
One method is disclosed in Japanese Unexamined Patent Application Publication No. 59-218483. In this method, a transmission/reflection mode is provided by disposing a transflective film, which is a thin metallic film, between a backlight and a liquid crystal layer. However, in principle, in this transmission/reflection mode, the transmission mode and the reflection mode cannot be optimized at the same time. More specifically, since the same liquid crystal layer is used in the transmission mode and the reflection mode, when optical designing is carried out by giving precedence to the transmission mode, the visibility in the reflection mode is reduced, whereas, when optical designing is carried out by giving precedence to the reflection mode, optical characteristics during transmission of light become poor.
The other method is disclosed in Japanese Unexamined Patent Application Publication No. 11-242226. In this method, a reflection/transmission mode is achieved by separating a reflection section and a transmission section within a pixel area. The orientation states of liquid crystals of the transmission section and the reflection section are made different, so that thought is put in obtaining good optical characteristics during reflection and transmission of light.
More specifically, optimization is achieved by changing the thicknesses of the liquid crystals at the reflection section and at the transmission section within a pixel. In other words, by setting the phase differences of wavelengths in the visible wavelength region when the voltage is turned on and when it is turned off at xcex/2 in the transmission section and at xcex/4 in the reflection section, a high reflection ratio, a high transmission ratio, and high contrast are achieved at both the reflection section and the transmission section. That is, the thickness of the liquid crystals at the transmission section is twice the thickness of the liquid crystals at the reflection section.
Japanese Patent Application Nos. 9-359036 and 10-364247 introduce as liquid crystal modes, such as: (1) a guest host mode in which doping of a dichroic coloring material is carried out, (2) a twist orientation mode, and (3) a homogeneous orientation mode. Emphasis is put in making the thickness of the portion of the liquid crystal layer at the transmission section and that of the portion of the liquid crystal layer at the reflection region different from each other.
Here, by changing the thickness of the portion of the liquid crystal layer at the transmission region and the thickness of the portion of the liquid crystal layer at the reflection region, panels in the liquid crystal modes (1), (2), and (3) were experimentally produced and inspected. In particular, the mode (3) was studied at great length because it was closest to being used for practical purposes.
A reflective-and-transmissive liquid crystal device in which the guest host mode (mode (1)) was used was produced and evaluated. The result of the evaluation makes it possible to confirm that the reflection ratio and the transmission ratio are high because a polarizing plate is not used. However, it has been found that contrast is not sufficient because the black level cannot be made sufficiently low due to insufficient ratio between the two colors of the dichroic coloring material.
The twist orientation mode (mode (2)) was used in a reflective-and-transmissive liquid crystal display device in order to produce and evaluate a panel. The result of the evaluation showed that, when a twist orientation process is carried out, it is difficult to control the orientation of the liquid crystals at the boundary between the transmission region and the reflection region.
An evaluation in the homogeneous mode (mode (3)) was carried out. In the homogeneous mode, liquid crystal molecules are orientated horizontal (parallel) to a substrate. The directions of orientation of the liquid crystals within a plane are often controlled to one direction by rubbing or the like. The vertical rubbing directions are anti-parallel directions. In the case where a horizontal mode is used, when there is a step at the transmission section and the reflection section, there is the advantage that retardation can be precisely obtained in proportion to the difference in the level. More specifically, if the thickness of the portion of the liquid crystal layer at the reflection section is made half that of the portion of the liquid crystal layer at the transmission section, the difference in retardation is halved. In the horizontal mode, liquid crystal material whose dielectric anisotropy is positive is used.
The inventor et al. repeatedly experimentally produced and inspected the panel that was optically designed so that the reflective-and-transmissive device whose liquid crystals were horizontally oriented was in a normally white mode. The results showed that the reflective-and-transmissive device of this type has the following disadvantages.
It has been made clear that, in the horizontal mode, even if a voltage of 5 volts is applied to the liquid crystal panel, the liquid crystals are not completely vertical, so that retardation remained. The remaining retardation is of the order of approximately 60 mm.
It is possible to increase contrast by inserting a retardation film equivalent to the remaining retardation between polarizing plates in order to decrease the black level. However, since the remaining retardation depends upon the thickness of a cell, it is not constant. In addition, the retardation film is produced by subjecting a polymer to centrifugal processing, so that there are variations in the retardation value. In other words, the retardation value is not constant. Therefore, it is very difficult to completely cancel the variations in these retardation values.
In addition, wavelength dispersion of the refractive index of the liquid crystal material and the wavelength dispersion of the retardation film can never become exactly the same.
Due to these reasons, the invention et al. have concluded that it is very difficult to obtain high contrast in the horizontal mode.
Even if high contrast is obtained, the necessity of using one retardation film for a dark display leads to increased costs. In particular, production process of a small retardation film of a few tens of millimeters in size becomes a delicate process, thereby resulting in high costs.
The viewing angle was also evaluated. In the horizontal orientation mode, the orientation of the liquid crystals is controlled in one direction by rubbing or the like. Therefore, the liquid crystal molecules stand up in one direction.
When the panel is viewed from the standing-up direction of the liquid crystal molecules, the retardation of the liquid crystals becomes large. When it is viewed from a direction opposite to this direction (180-degree rotation), however, the retardation of the liquid crystals becomes small. Accordingly, in a uniaxial orientation mode where the liquid crystal molecules are tilted only in one direction by an electrical field, the effective retardation differs greatly depending on the viewing direction.
It goes without saying that differences in retardation depending on the viewing direction cause differences in image depending upon the viewing direction. In other words, when the retardation is greatly dependent upon the viewing angle, the viewing angle characteristic of the visibility of the panel naturally becomes poor. The inventor et al. have confirmed that, even in the experiment, the viewing angle characteristic in the horizontal mode is very poor in principle. This principle results from the reasons given above. Accordingly, the inventor et al. have realized that the horizontal mode is not suitable for use in a reflective-and-transmissive liquid crystal display device.
Accordingly, the present invention attempts to overcome the problems of a reflective-and-transmissive liquid crystal display device, and to make it possible to, in a transmission display mode, provide high contrast and a high transmission ratio, and, in a reflection display mode, provide high contrast and a high reflection ratio. In addition, the present invention tries to make it possible to achieve a wide viewing angle in both the transmission mode and the reflection mode.
To these ends, according to a basic form of the present invention, there is provided a liquid crystal display device comprising a first substrate capable of transmitting light therethrough; a second substrate at which a pixel including a reflection region and a transmission region is formed; and a liquid crystal section held by the first substrate and the second substrate that are bonded together through a gap. In the liquid crystal display device, the liquid crystal section is oriented vertically with respect to the substrates when no voltage is applied thereto. In addition, a pair of retardation films are disposed on both sides of the vertically oriented liquid crystal section, and have symmetrical phase characteristics over a visible wavelength region. In one form of the basic form, lagging phase axes of the pair of retardation films intersect each other, with an intersecting angle thereof being set in a range of 90xc2x0xc2x110xc2x0. In another form of the basic form, relative shifts in phase differences of the pair of retardation films are controlled within a range of xc2x130 nm.
In still another form of the basic form, a thickness of a portion of the liquid crystal section at the transmission region is twice a thickness of a portion of the liquid crystal section at the reflection region. In still another form of the basic form, at least one of the first and second substrates includes a step for changing the thickness of the liquid crystal section by the transmission region and the reflection region. When at least one of the first and second substrates includes a step for changing the thickness of the liquid crystal section by the transmission region and the reflection region, the step may be a recess formed by selectively removing an insulating film formed at at least one of the first and second substrates from the transmission region. When the step is a recess formed by selectively removing an insulating film formed at at least one of the first and second substrates from the transmission region, the liquid crystal display device may be constructed so that the orientation of the liquid crystal section is controlled using the recess, and the vertical orientation thereof switches to a multiaxial orientation thereof by application of a voltage. When the liquid crystal display device is constructed so that the orientation of the liquid crystal section is controlled using the recess, and the vertical orientation thereof switches to a multiaxial orientation thereof by application of a voltage, the recess may be symmetrical about a geometrical center point thereof.
In still another form of the basic form, the liquid crystal display device may be constructed so that the liquid crystal section has a chiral agent added thereto, and the vertical orientation thereof changes to a twist orientation thereof by application of a voltage. In still another form of the basic form, the liquid crystal display device may be constructed so that a portion of the liquid crystal section at the transmission region is oriented multiaxially by application of a voltage, and a portion of the liquid crystal section at the reflection region is oriented uniaxially by application of a voltage. When the liquid crystal display device is constructed so that a portion of the liquid crystal section at the transmission region is oriented multiaxially by application of a voltage, and a portion of the liquid crystal section at the reflection region is oriented uniaxially by application of a voltage, the liquid crystal section may be controlled so as to be oriented multiaxially using an electrode slit or a columnar member formed at the transmission region. When the liquid crystal section is controlled so as to be oriented multiaxially using an electrode slit or a columnar member formed at the transmission region, the columnar member may also serve as a spacer for restricting the gap between the first and second substrates at a constant size. In still another form of the basic form, the reflection region is subjected to rubbing for uniaxially orienting the liquid crystal section, while the transmission region is not subjected to rubbing. In still another form of the basic form, by selective irradiation using ultraviolet light, a difference is produced between a surface state of the transmission region and a surface state of the reflection region in order to cause the state of orientation of the liquid crystal section to differ by the transmission region and the reflection region.
The inventor et al. have studied various liquid crystal modes as means for overcoming the above-described problems. The results showed that the vertical orientation mode in which liquid crystal molecules are oriented perpendicular to a substrate is an optimal mode. The liquid crystal material used in this mode has a characteristic in which, whereas the refractive index anisotropy is parallel to a long axis direction of the liquid crystal, the dielectric anisotropy is orthogonal to the long axis direction of the liquid crystal. It has been found that high contrast, high transmission ratio/high reflection ratio, and a wide viewing angle are obtained in both reflection display and transmission display by optimizing the vertical orientation feature and optical designing. The designing method and principle are as described above. In other words, it is important that the pair of retardation films which are provided on both sides of a liquid crystal panel in the vertical orientation mode have symmetrical phase characteristics over the visible wavelength range. More specifically, the lagging phase axes of the pair of retardation films intersect each other, with the angle of intersection being set within the range of 90xc2x0xc2x110xc2x0. Relative shifts in the phase differences of the pair of retardation films are controlled within the range of xc2x130 nm.
It is desirable that the display in the transmission mode and the reflection mode be carried out by obtaining, in particular, a cell structure where the thickness of the portion of the liquid crystal section at the transmission region is approximately twice that of the portion of the liquid crystal section at the reflection region by making use of vertical orientation. When an electrical field is turned off, the liquid crystal is vertically oriented, so that retardation does not occur. When an electrical field is turned on, the liquid crystal is tilted, so that retardation occurs. The thickness of the cell is provided and the step that is formed at the reflection region and the transmission region is designed so that this retardation value is xcex/2 in the transmission region and xcex/4 in the reflection region.